Transformer is a device based on principle of mutual induction, it consists of primary and secondary coils. The given source of emf say AC mains is always connected to the primary coil. When alternating current flows through the primary coil, then in each cycle of current the core is magnetised once in one direction and once in opposite direction. Hence, alternating magnetic flux is produced. By principle of mutual induction an alternating emf of same frequency is induced in the secondary coil. Hence, frequency remains unchanged in a transformer, while voltage is stepped up or down, with varying current.